emacs: update notmuch-show-get-filename to handle duplicates

This fixes one of the bugs reported by Gregor in [1].

[1]: id:87edxw8jp4.fsf@no.workgroup

emacs/notmuch-show.el

@@ -1811,10 +1811,10 @@ current thread."
 
 ;; dme: Would it make sense to use a macro for many of these?
 
-;; XXX TODO figure out what to do about multiple filenames
 (defun notmuch-show-get-filename ()
   "Return the filename of the current message."
-  (car (notmuch-show-get-prop :filename)))
+  (let ((duplicate (or (notmuch-show-get-prop :duplicate) 1)))
+    (nth (1- duplicate) (notmuch-show-get-prop :filename))))
 
 (defun notmuch-show-get-header (header &optional props)
   "Return the named header of the current message, if any."

test/T450-emacs-show.sh

@@ -352,7 +352,6 @@ test_expect_equal "$output" "$subject"
 
 FILE3=$(notmuch search --output=files --duplicate=3 "id:${ID3}")
 test_begin_subtest "duplicate=3, stash"
-test_subtest_known_broken
 test_emacs_expect_t \
 	"(notmuch-show \"id:${ID3}\")
 	 (notmuch-show-choose-duplicate 3)